Method of soldering TI containing alloys

ABSTRACT

An improved guidewire for advancing a catheter within a body lumen which has a high strength proximal portion, a distal portion formed of superelastic alloy and a connector formed of superelastic alloy to provide torque transmitting coupling between the distal end of the proximal portion and the proximal end of the distal portion. The superelastic alloy elements are preferably cold worked and then heat treated at a temperature well above the austenite-to-martensite transformation temperature, while being subjected to longitudinal stresses equal to about 5 to about 50% of the room temperature yield stress to impart to the metal a straight &#34;memory.&#34; The guiding member using such improved material exhibits a stress induced austenite-to-martensite phase transformation at an exceptionally high constant yield strength of at least 70 ksi for solid members and at least 50 ksi for tubular members with a broad recoverable strain of at least about 4% during the phase transformation.

BACKGROUND OF THE INVENTION

This invention relates to the field of medical devices, and moreparticularly to guiding means such as a guidewire for advancing acatheter within a body lumen in a procedure such as percutaneoustransluminal coronary angioplasty (PTCA).

In a typical PTCA procedure a guiding catheter having a preformed distaltip is percutaneously introduced into the cardiovascular system of apatient in a conventional Seldinger technique and advanced therein untilthe distal tip of the guiding catheter is seated in the ostium of adesired coronary artery. A guidewire is positioned within an inner lumenof a dilatation catheter and then both are advanced through the guidingcatheter to the distal end thereof. The guidewire is first advanced outof the distal end of the guiding catheter into the patient's coronaryvasculature until the distal end of the guidewire crosses a lesion to bedilated, then the dilatation catheter having an inflatable balloon onthe distal portion thereof is advanced into the patient's coronaryanatomy over the previously introduced guidewire until the balloon ofthe dilatation catheter is properly positioned across the lesion. Oncein position across the lesion, the balloon is inflated to apredetermined size with radiopaque liquid at relatively high pressures(e.g., greater than 4 atmospheres) to compress the arterioscleroticplaque of the lesion against the inside of the artery wall and tootherwise expand the inner lumen of the artery. The balloon is thendeflated so that blood flow is resumed through the dilated artery andthe dilatation catheter can be removed therefrom.

Conventional guidewires for angioplasty and other vascular proceduresusually comprise an elongated core member with one or more taperedsections near the distal end thereof and a flexible body such as ahelical coil disposed about the distal portion of the core member. Ashapable member, which may be the distal extremity of the core member ora separate shaping ribbon which is secured to the distal extremity ofthe core member extends through the flexible body and is secured to arounded plug at the distal end of the flexible body. Torquing means areprovided on the proximal end of the core member to rotate, and therebysteer, the guidewire while it is being advanced through a patient'svascular system.

Further details of dilatation catheters, guidewires, and devicesassociated therewith for angioplasty procedures can be found in U.S.Pat. No. 4,323,071 (Simpson-Robert); U.S. Pat. No. 4,439,185(Lundquist): U.S. Pat. No. 4,516,972 (Samson); U.S. Pat. No. 4,538,622(Samson, et al.); U.S. Pat. No. 4,554,929 (Samson, et al.); U.S. Pat.No. 4,616,652 (Simpson); and U.S. Pat. No. 4,638,805 (Powell) which arehereby incorporated herein in their entirety by reference thereto.

Steerable dilatation catheters with fixed, built-in guiding members,such as described in U.S. Pat. No. 4,582,181 (now Re 33,166) arefrequently used because they have lower deflated profiles thanconventional over-the-wire dilatation catheters and a lower profileallows the catheter to cross tighter lesions and to be advanced muchdeeper into a patient's coronary anatomy.

A major requirement for guidewires and other guiding members, whetherthey be solid wire or tubular members, is that they have sufficientcolumn strength to be pushed through a patient's vascular system orother body lumen without kinking. However, they must also be flexibleenough to avoid damaging the blood vessel or other body lumen throughwhich they are advanced. Efforts have been made to improve both thestrength and flexibility of guidewires to make them more suitable fortheir intended uses, but these two properties are for the most partdiametrically opposed to one another in that an increase in one usuallyinvolves a decrease in the other.

The prior art makes reference to the use of alloys such as Nitinol(Ni-Ti alloy) which have shape memory and/or superelasticcharacteristics in medical devices which are designed to be insertedinto a patient's body. The shape memory characteristics allow thedevices to be deformed to facilitate their insertion into a body lumenor cavity and then be heated within the body so that the device returnsto its original shape. Superelastic characteristics on the other handgenerally allow the metal to be deformed and restrained in the deformedcondition to facilitate the insertion of the medical device containingthe metal into a patient's body, with such deformation causing the phasetransformation. Once within the body lumen the restraint on thesuperelastic member can be removed, thereby reducing the stress thereinso that the superelastic member can return to its original undeformedshape by the transformation back to the original phase.

Alloys have shape memory/superelastic characteristics generally have atleast two phases, a martensite phase, which has a relatively low tensilestrength and which is stable at relatively low temperatures, and anaustenite phase, which has a relatively high tensile strength and whichis stable at temperatures higher than the martensite phase.

Shape memory characteristics are imparted to the alloy by heating themetal at a temperature above which the transformation from themartensite phase to the austenite phase is complete, i.e., a temperatureabove which the austenite phase is stable. The shape of the metal duringthis heat treatment is the shape "remembered." The heat-treated metal iscooled to a temperature at which the martensite phase is stable, causingthe austenite phase to transform to the martensite phase. The metal inthe martensite phase is then plastically deformed, e.g., to facilitatethe entry thereof into a patient's body. Subsequent heating of thedeformed martensite phase to a temperature above the martensite toaustenite transformation temperature causes the deformed martensitephase to transform to the austenite phase and during this phasetransformation the metal reverts back to its original shape.

The prior methods of using the shape memory characteristics of thesealloys in medical devices intended to be placed within a patient's bodypresented operational difficulties. For example, with shape memoryalloys having a stable martensite temperature below body temperature, itwas frequently difficult to maintain the temperature of the medicaldevice containing such an alloy sufficiently below body temperature toprevent the transformation of the martensite phase to the austenitephase when the device was being inserted into a patient's body. Withintravascular devices formed of shape memory alloys havingmartensite-to-austenite transformation temperatures well above bodytemperature, the devices could be introduced into a patient's body withlittle or no problem, but they had to be heated to themartensite-to-austenite transformation temperature which was frequentlyhigh enough to cause tissue damage and very high levels of pain.

When stress is applied to a specimen of a metal such as Nitinolexhibiting superelastic characteristics at a temperature at or abovewhich the transformation of martensite phase to the austenite phase iscomplete, the specimen deforms elastically until it reaches a particularstress level where the alloy then undergoes a stress-induced phasetransformation from the austenite phase to the martensite phase. As thephase transformation proceeds, the alloy undergoes significant increasesin strain but with little or no corresponding increases in stress. Thestrain increases while the stress remains essentially constant until thetransformation of the austenite phase to the martensite phase iscomplete. Thereafter, further increase in stress is necessary to causefurther deformation. The martensitic metal first yields elastically uponthe application of additional stress and then plastically with permanentresidual deformation.

If the load on the specimen is removed before any permanent deformationhas occurred, the martensitic specimen will elastically recover andtransform back to the austenite phase. The reduction in stress firstcauses a decrease in strain. As stress reduction reaches the level atwhich the martensite phase transforms back into the austenite phase, thestress level in the specimen will remain essentially constant (butsubstantially less than the constant stress level at which the austenitetransforms to the martensite) until the transformation back to theaustenite phase is complete, i.e., there is significant recovery instrain with only negligible corresponding stress reduction. After thetransformation back to austenite is complete, further stress reductionresults in elastic strain reduction. This ability to incur significantstrain at relatively constant stress upon the application of a load andto recover from the deformation upon the removal of the load is commonlyreferred to as superelasticity or pseudoelasticity.

The prior art makes reference to the use of metal alloys havingsuperelastic characteristics in medical devices which are intended to beinserted or otherwise used within a patient's body. See for example,U.S. Pat. No. 4,665,905 (Jervis) and U.S. Pat. No. 4,925,445 (Sakamoto,et al.).

The Sakamoto, et al. patent discloses the use of a nickel-titaniumsuperelastic alloy in an intravascular guidewire which could beprocessed to develop relatively high yield strength levels. However, atthe relatively high yield stress levels which cause theaustenite-to-martensite phase transformation characteristic of thematerial, it did not have a very extensive stress-induced strain rangein which the austenite transforms to martensite at relative constantstress. As a result, frequently as the guidewire was being advancedthrough a patient's tortuous vascular system, it would be stressedbeyond the superelastic region, i.e., develop a permanent set or evenkink which can result in tissue damage. This permanent deformation wouldgenerally require the removal of the guidewire and the replacementthereof with another.

Products of the Jervis patent on the other hand had extensive strainranges, i.e., 2 to 8% strain, but the relatively constant stress levelat which the austenite transformed to martensite was very low, e.g., 50ksi.

In copending application Ser. No. 07/629,381, filed Dec. 18, 1990,entitled Superelastic Guiding Member, guide wires or guiding members aredescribed which have at least a solid or tubular portion thereofexhibiting superelastic characteristics including an extended strainregion over a very high, relatively constant high stress level whicheffects the austenite transformation to martensite. While the propertiesof the guidewire formed of the superelastic material were veryadvantageous, it was found that the guidewires and guiding membersformed of materials having superelastic characteristics did not haveoptimum push and torque characteristics.

SUMMARY OF THE INVENTION

The present invention is directed to improve guidewires or guidingmembers, wherein the distal portion is provided with superelasticcharacteristics resulting from the stress-induced transformation ofaustenite to martensite and wherein the proximal portion is providedwith high strength elastic materials.

The guidewire or guiding member of the invention has a high strengthproximal section with a high strength distal section with superelasticproperties and a connector element between the proximal and distalsections which superelastic properties to provide a smooth transitionbetween the proximal and the distal sections. In a presently preferredembodiment the guidewire or guiding member has a solid core distalsection formed of superelastic materials such as NiTi type alloys andthe connector is a hollow tubular shaped member which has an innerpassageway adapted to receive the proximal end of the solid core distalsection.

The superelastic distal core member and the hollow connector of theinvention exhibit stress-induced phase transformation at bodytemperature (about 37° C.) at a stress level well above about 50 ksi,preferably above 70 ksi and in many cases above about 90 ksi. Thecomplete stress-induced transformation of the austenite phase to themartensite phase causes a strain in the specimen of at least about 4%,preferably over 5%. The region of phase transformation resulting fromstress preferably begins when the specimen has been strained about 2 to3% at the onset of the phase change from austenite to martensite andextends to about 7 to about 9% strain at the completion of the phasechange. The stress and strain referred to herein is measured by tensiletesting. The stress-strain relationship determined by applying a bendingmoment to a cantilevered specimen is slightly different from therelationship determined by tensile testing because the stresses whichoccur in the specimen during bending are not as uniform as they are intensile testing. There is considerably less change in stress during thephase transformation than either before or after the stress-inducedtransformation. The stress level is relatively constant within thetransformation period.

The portions of the guiding member having superelastic properties arepreferably formed from an alloy consisting essentially of about 30 toabout 52% titanium and the balance nickel and up to 10% of one or moreadditional alloying elements. Such other alloying elements may beselected from the group consisting of up to 3% each of iron, cobalt,platinum, palladium and chromium and up to about 10% copper andvanadium. As used herein all references to percent composition areatomic percent unless otherwise noted.

To form the elongated superelastic portion of the guiding member,elongated solid rod or tubular stock of the preferred alloy material isfirst cold worked, preferably by drawing, to effect a size reduction ofabout 30% to about 70% in the transverse cross-section thereof. Thecold-worked material may then be given a memory imparting heat treatmentat a temperature of about 350° to 600° C. for about 0.5 to about 60minutes, while maintaining a longitudinal stress on the elongatedportion equal to about 5% to about 50%, preferably about 10% to about30%, of the yield stress of the material (as measured at roomtemperature). This thermomechanical processing imparts a straight"memory" to the superelastic portion and provides a relatively uniformresidual stress in the material. Another method involves mechanicallystraightening the wire after the cold work and then heat treating thewire at temperatures between about 300° and about 450° C., preferablyabout 330° to about 400° C. The latter treatment provides substantiallyhigher tensile properties. The cold-worked and heat-treated alloymaterial has an austenite finish transformation temperature less thanbody temperature and generally about -10° C. to about 30° C. For moreconsistent final properties, it is preferred to fully anneal the solidrod or tubular stock prior to cold work so that the material will alwayshave the same metallurgical structure at the start of the cold workingand so that it will have adequate ductility for subsequent cold working.It will be appreciated by those skilled in the art that means of coldworking the metal other than drawing, such as rolling or swaging, can beemployed. The constant yield stress levels for solid products have beenfound to be slightly lower than the levels for solid products. Forexample, superelastic wire material of the invention will have aconstant stress level usually above about 70 ksi, preferably above about90 ksi, whereas, superelastic tubing material will have a constantstress level of above 50 ksi, preferably above about 70 ksi. Theultimate tensile strength of both forms of the material is well above200 ksi with an ultimate elongation at failure of about 15%.

The elongated superelastic members of the invention exhibitstress-induced austenite-to-martensite phase transformation over a broadregion of strain at very high, relatively constant stress levels. As aresult a guiding member having a distal portion formed of this materialis very flexible, it can be advanced through very tortuous passagewayssuch as a patient's coronary vasculature with little risk that thesuperelastic portion of the guiding member will develop a permanent setand at the same time it will effectively transmit the torque appliedthereto without causing the guiding member to whip. The high strengthproximal portion of the guidewire or guiding member provides excellentpushability and torquability to the guidewire or guiding member.

These and other advantages of the invention will become more apparentfrom the following detailed description thereof when taken inconjunction with the following exemplary drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of a guidewire which embodies features ofthe invention.

FIG. 2 is a schematic, graphical illustration of the stress-strainrelationship of superelastic material.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a guidewire 10 embodying features of the inventionthat is adapted to be inserted into a patient's body lumen, such as anartery. The guidewire 10 comprises an elongated, relatively highstrength proximal portion 11, a relatively short distal portion 12 whichis formed substantially of superelastic alloy material and a connectorelement 13 which is formed substantially of superelastic alloy materialand which connects the proximal end of the distal portion 12 to thedistal end of the proximal portion 11 into a torque transmittingrelationship. The distal portion 12 has at least one tapered section 14which becomes smaller in the distal direction. The connector element 13is a hollow tubular shaped element having an inner lumen extendingtherein which is adapted to receive the proximal end 15 of the distalportion 12 and the distal end 16 of the proximal portion 11. The ends 15and 16 may be press fit into the connector element or they may besecured therein by crimping or swaging the connector or by means such asa suitable adhesive or by welding, brazing or soldering.

A helical coil 17 is disposed about the distal portion 12 and has arounded plug 18 on the distal end thereof. The coil 17 is secured to thedistal portion 12 at proximal location 20 and at intermediate location21 by a suitable solder. A shaping ribbon 22 is secured by its proximalend to the distal portion 12 at the same location 21 by the solder andby the distal end thereof to the rounded plug 18 which is usually formedby soldering or welding the distal end of the coil 17 to the distal tipof the shaping ribbon 22. Preferably, the most distal section 24 of thehelical coil 17 is made of radiopaque metal such as platinum orplatinum-nickel alloys to facilitate the observation thereof while it isdisposed within a patient's body. The most distal section 24 should bestretched about 10 to about 30%.

The most distal part 25 of the distal portion 12 is flattened into arectangular section and preferably provided with a rounded tip 26, e.g.,solder to prevent the passage of the most distal part through thespacing between the stretched distal section 24 of the helical coil 17.

The exposed portion of the elongated proximal portion 11 should beprovided with a coating 27 of lubricous material such aspolytetrafluoroethylene (sold under the trademark Teflon by du Pont, deNemours & Co.) or other suitable lubricous coatings such as thepolysiloxane coatings disclosed in co-pending application Ser. No.559,373, filed Jul. 24, 1990, which is hereby incorporated by reference.

The elongated proximal portion 11 of the guidewire 10 is generally about130 to about 140 cm in length with an outer diameter of about 0.006 to0.018 inch for coronary use. Larger diameter guidewires may be employedin peripheral arteries and other body lumens. The lengths of the smallerdiameter and tapered sections can range from about 2 to about 20 cm,depending upon the stiffness or flexibility desired in the finalproduct. The helical coil 17 is about 20 to about 45 cm in length, hasan outer diameter about the same size as the diameter of the elongatedproximal portion 11, and is made from wire about 0.002 to 0.003 inch indiameter. The shaping ribbon 22 and the flattened distal section 26 ofdistal portion 12 have rectangular transverse cross-sections whichusually have dimensions of about 0.001 by 0.003 inch.

The superelastic members of the invention, i.e., the distal portion 12and the connector 13, are preferably made of an alloy materialconsisting of about 30 to about 52% titanium and the balance nickel andup to 10% of one or more other alloying elements. The other alloyingelements may be selected from the group consisting of iron, cobalt,vanadium, platinum, palladium and copper. The alloy can contain up toabout 10% copper and vanadium and up to 3% of the other alloyingelements. The addition of nickel above the equiatomic amounts withtitanium and the other identified alloying elements increase the stresslevels at which the stress-induced austenite-to-martensitetransformation occurs and ensure that the temperature at which themartensite phase transforms to the austenite phase is well below humanbody temperature so that austenite is the only stable phase at bodytemperature. The excess nickel and additional alloying elements alsohelp to provide an expanded strain range at very high stresses when thestress-induced transformation of the austenite phase to the martensitephase occurs.

A presently preferred method for making the final configuration of thesuperelastic portions of the guiding member is to cold work, preferablyby drawing, a rod or tubular member having a composition according tothe relative proportions described above and then heat treating thecold-worked product while it is under stress to impart a shape memorythereto. Typical initial transverse dimensions of the rod or the tubularmember are about 0.045 inch and about 0.25 inch respectively. If thefinal product is to be tubular, a small diameter ingot, e.g., 0.25 toabout 1.5 inch in diameter and 5 to about 30 inches in length, may beformed into a hollow tube by extruding or by machining a longitudinalcenter hole therethrough and grinding the outer surface thereof smooth.Before drawing the solid rod or tubular member, it is preferablyannealed at a temperature of about 500° to about 750° C., typicallyabout 650° C., for about 30 minutes in a protective atmosphere such asargon to relieve essentially all internal stresses. In this manner allof the specimens start the subsequent thermomechanical processing inessentially the same metallurgical condition so that products withconsistent final properties are obtained. Such treatment also providesthe requisite ductility for effective cold working.

The stressed relieved stock is cold worked by drawing to effect areduction in the cross-sectional area thereof of about 30 to 70%. Themetal is drawn through one or more dies of appropriate inner diameterwith a reduction per pass of about 10 to 50%. Other forms of coldworking can be employed such as swaging.

Following cold work, the drawn wire or hollow tubular product is heattreated at a temperature between about 350° and about 600° C. for about0.5 to about 60 minutes. Preferably, the drawn wire or hollow tubularproduct is simultaneously subjected to a longitudinal stress betweenabout 5% and about 50%, preferably about 10% to about 30% of the tensilestrength of the material (as measured at room temperature) in order toimpart a straight "memory" to the metal and to ensure that any residualstresses therein are uniform. This memory-imparting heat treatment alsofixes the austenite-martensite transformation temperature for thecold-worked metal. By developing a straight "memory" and maintaininguniform residual stresses in the superelastic material, there is littleor no tendency for a guidewire made of this material to whip when it istorqued within a patient's blood vessel.

An alternate model for imparting a straight memory to the cold-workedmaterial includes mechanically straightening the wire or tube and thensubjecting the straightened wire to a memory-imparting heat treatment ata temperature of about 300° to about 450° C., preferably about 330° toabout 400° C. The latter treatment provides substantially improvedtensile properties, but it is not very effective on materials which havebeen cold worked above 55%, particularly above 60%. Materials producedin this manner exhibit stress-induced austenite to martensite phasetransformation at very high levels of stress but the stress during thephase transformation is not nearly as constant as the previouslydiscussed method. Conventional mechanical straightening means can beused such as subjecting the material to sufficient longitudinal stressto straighten it.

FIG. 2 illustrates an idealized stress-strain relationship of an alloyspecimen having superelastic properties as would be exhibited upontensile testing of the specimen. The line from point A to point Bthereon represents the elastic deformation of the specimen. After pointB the strain or deformation is no longer proportional to the appliedstress and it is in the region between point B and point C that thestress-induced transformation of the austenite phase to the martensitephase begins to occur. There can be an intermediate phase developed,sometimes called the rhombohedral phase, depending upon the compositionof the alloy. At point C the material enters a region of relativelyconstant stress with significant deformation or strain. It is in thisregion that the transformation from austenite to martensite occurs. Atpoint D the transformation to the martensite phase due to theapplication of tensile stress to the specimen is substantially complete.Beyond point D the martensite phase begins to deform, elastically atfirst, but, beyond point E, the deformation is plastic or permanent.

When the stress applied to the superelastic metal is removed, the metalwill recover to its original shape, provided that there was no permanentdeformation to the martensite phase. At point F in the recovery process,the metal begins to transform from the stress-induced, unstablemartensite phase back to the more stable austenite phase. In the regionfrom point G to point H, which is also an essentially constant stressregion, the phase transformation from martensite back to austenite isessentially complete. The line from point I to the starting point Arepresents the elastic recovery of the metal to its original shape.

Because of the extended strain range under stress-induced phasetransformation which is characteristic of the superelastic materialdescribed herein, a guidewire having a distal portion made at least insubstantial part of such material can be readily advanced throughtortuous arterial passageways. When the distal end of the guidewireengages the wall of a body lumen such as a blood vessel, it willsuperelastically deform as the austenite transforms to martensite. Uponthe disengagement of the distal end of the guidewire from the vesselwall, the stress is reduced or eliminated from within the superelasticportion of the guidewire and it recovers to its original shape, i.e.,the shape "remembered" which is preferably straight. The straight"memory" in conjunction with little or no nonuniform residuallongitudinal stresses within the guidewire prevent whipping of theguidewire when it is torqued from the proximal end thereof. Moreover,due to the very high level of stress needed to transform the austenitephase to the martensite phase, there is little chance for permanentdeformation of the guidewire or the guiding member when it is advancedthrough a patient's artery.

The tubular connector formed of superelastic alloy material provides asmooth transition between the high strength proximal portion and therelatively short distal section and retains a torque transmittingrelationship between these two portions.

The present invention provides guidewires which have superelasticcharacteristics to facilitate the advancing thereof in a body lumen. Theguiding members exhibit extensive, recoverable strain resulting fromstress induced phase transformation of austenite to martensite atexceptionally high stress levels which greatly minimizes the risk ofdamage to arteries during the advancement therein.

The Nitinol hypotubing from which the connector is formed generally mayhave an outer diameter from about 0.006 inch to about 0.02 inch withwall thicknesses of about 0.001 to about 0.004 inch. A presentlypreferred superelastic hypotubing for the connecting member has an outerdiameter of about 0.014 inch and a wall thickness of about 0.002 inch.

Superelastic NiTi alloys, such as those described herein, are verydifficult to solder due to the formation of a tenacious, naturallyoccurring oxide coating which prevents the molten solder from wettingthe surface of the alloy in a manner necessary to develop a sound,essentially oxide free, soldered joint. It has been found that by firsttreating the surface of the refractory superelastic alloy with moltenalkali metal hydroxide, e.g., sodium, potassium, lithium or mixturesthereof to form a nascent alloy surface and then pretinning with asuitable solder such as a gold-tin solder without contacting air, thatthe superelastic piece can be readily soldered in a conventional manner.A presently preferred alkali metal hydroxide is a mixture of about 59% Kand about 41% Na. The solder may contain from about 60 to about 85% goldand the balance tin, with the presently preferred solder containingabout 80% gold and about 20% tin. In a presently preferred procedure amultilayered bath is provided with an upper layer of molten alkali metalhydroxide and a lower layer of molten gold-tin solder. The part of thesuperelastic distal portion, which is to be soldered, is thrust into themultilayered bath through the upper surface of the molten alkali metalhydroxide which removes the oxide coating, leaving a nascent metal alloysurface, and then into the molten solder which wets the nascent metalsurface. When the solder solidifies upon removal from the molten solderinto a thin coating on the metal alloy surface, the underlying alloysurface is protected from on oxygen-containing atmosphere. Any of thealkali metal hydroxide on the surface of the solder can be easilyremoved with water without detrimentally affecting either the pretinnedlayer or the underlying alloy surface. The superelastic member is thenready for conventional soldering. The procedure may be employed toprepare other metal alloys having significant titanium levels forsoldering.

The high strength proximal portion of the guidewire generally issignificantly stronger, i.e., higher ultimate tensile strength, than thesuperelastic distal portion. Suitable high strength materials include304 stainless steel which is a conventional material in guidewireconstruction.

While the above description of the invention is directed to presentlypreferred embodiments, various modifications and improvements can bemade to the invention without departing therefrom.

What is claimed is:
 1. A method for making an intravascular guidewirecomprising:a) providing a core member formed of an alloy containingsignificant amounts of titanium; b) treating at least a portion of thecore member to remove surface oxide material therefrom; c) coating thethus treated portion of the core member to prevent surface reoxidationthereof; and d) soldering a helical coil disposed about the thus treatedportion of the core member to the treated portion of the core member. 2.The method of claim 1 wherein a solder is employed comprising a gold-tinalloy.
 3. The method of claim 1 wherein the surface oxide is removed bytreating the surface of the core member with a molten alkali metalhydroxide.
 4. The method of claim 3 wherein the surface of the coremember is coated with a solder after removing surface oxide therefrom toprevent the reoxidation of said surface.
 5. The method of claim 4wherein a bath of molten solder is maintained in a container with alayer of molten alkali metal hydroxide on an upper surface of the moltensolder and the core member is thrust through the molten alkali metalhydroxide to remove surface oxide therefrom and then into the underlyingmolten solder to coat the core member with the solder and thensolidifying the molten solder on the surface of the core member.